The present invention concerns a method for coating the cathode of an electron gun with a thermionic emissive substance and an apparatus therefor.
Generally, the cathode of an electron gun is coated with alkaline earth metal carbonate as the thermionic emissive substance. More specifically, the cathode is made of a nickel cap welded onto a sleeve. The upper surface of the nickel cap is etched by a weak acid, and coated with a slurry composed of barium carbonate (BaCO3), strontium carbonate (SrCO3) and calcium carbonate (CaCO3) which is added with organic solvent and a binder, and ball-milled.
In the above process, the carbonate layer coated on the cathode must be oxidized by heating at about 950.degree. C. in order to dispose of additives such as the binder, because of which the cathode may be easily stripped of the coated layer, thereby increasing the fault ratio. Moreover, because the coated layer is adhered to the metal cap by the binder, the adhesive strength may not be improved, and pressurized air is employed to spray the coating substance, so that oil, water, etc. are introduced into the coated substance, thereby blackening the cathode or impairing the thermionic emission characteristic.
Besides, because the density of the coated layer may not be improved, the thickness of the layer must be increased, so that the thermal conduction to the outer part of the coated layer is slowed down and therefore, the ignition of the electron gun is also slowed down.